1. Field of the Invention
The invention relates generally to relative phase measurements and more specifically to receivers for determining phase differentials of RF signals.
2. Description of the Prior Art
Various electronic navigation systems are well known in the art. Loran systems, for instance, have found wide acceptance and are proved to be a valuable aid to navigation. More recently, a Global Positioning System (GPS) has been proposed wherein a constellation of satellites will be positioned in orbit around the Earth. Each satellite transmits data by via signals operating at two frequencies which are modulated by dual psuedorandum noise codes in clear/acquisition (CA) and precision (P) codes peculiar to that satellite. The modulation is structured to establish a spread spectrum transmission that can be identified and processed by the user to determine his position. Receivers for utilization and such systems are described, for instance, in U.S. Pat. No. 4,457,006 issued to Reuben E. Maine on June 26, 1984 entitled "Global Positioning System Receiver" and in U.S. Pat. No. 4,453,165 issued to Reuben E. Maine on June 5, 1984 entitled "Differential Doppler Receiver" both patents being assigned to the assignee of the instant invention.
The receiver disclosed in the former U.S. Application utilizes a phase lock loop to derive local oscillators for a double heterodyne receiver which converts IF signals from the earth's orbital satellite to audio frequency output signals. Since both local oscillators are locked to a common reference oscillator, the phase variation of the audio signal contains all the information impressed on the RF signals. In the latter U.S. application, the receiver deisclosed retains the advantages of prior GPS receivers and additionally accurately measures the Doppler frequency of RF signals from a transmitter moving with respect to the receiver by using a second transmitter having a stable and precisely known frequency as the reference. Signals from the two sources are processed in a differential mode to eliminate the need for a precise timing clock in the receiver.
The receiver of the present invention generates low frequency output signals from which the relative phase angles of an RF carrier signal received at separated antennas may be extracted.